The present invention relates to digital imaging devices and related methods, and more particularly, to the elimination of modulated light effects in rolling shutter CMOS sensor images.
Due to the nature of the electronic rolling shutter readout used in CMOS image sensors, significant horizontal banding can occur in images taken in the presence of modulated lighting, such as fluorescent lights, for example. This is illustrated in FIG. 1. This problem affects liveview, video, and still image captures taken without a mechanical shutter.
More particularly, most digital still cameras that use CMOS sensors employ a rolling shutter readout, which means that the exposure and readout of each image row is staggered or shifted in time from a previous row. Due to this readout method, liveview and video images exhibit significant horizontal banding when imaging scenes illuminated with modulated light, such as fluorescent lighting. Still images taken without a mechanical shutter also exhibit this horizontal banding. This is one of a number of reasons that have necessitated the need for a mechanical shutter in digital still cameras that use CMOS sensors.
The use of a mechanical shutter eliminates this problem in still images. However, the disadvantage of this is the cost of the shutter mechanism. This is not the only problem standing in the way of eliminating the mechanical shutter. Other reasons requiring a mechanical shutter are to provide the minimum exposure time required for strobe shots and to minimize motion distortion.
Non-SLR (single lens reflex) CMOS based digital still cameras that do not have a mechanical shutter exhibit this problem in still image captures. It is believed that the image banding problem exists on all non-SLR CMOS based digital still cameras that have video and/or liveview capability or that do not utilize a mechanical shutter for still image captures.
U.S. Pat. No. 6,771,305 issued to Smith et al., as evidenced by its Abstract, for example, discloses a “method for eliminating the flicker of a light source such as a fluorescent light.” This method captures images at an exposure time that is not synchronous to either of the two possible frequencies and then 1) performs a horizontal average of each line of the image (column 6, step 306), and 2) performs an FFT across the resulting averages (column 6, step 310). The results of the FFT are used to determine the light modulation frequency (column 6, step 312). The present application discloses different and improved techniques for eliminating the effects of modulated light sources on digital imaging devices such as digital cameras.
It would be desirable to be able to eliminate the effects of modulated light sources on digital cameras that use CMOS sensors employing a rolling shutter readout.